How much do I need to spend to get a preamp that sounds better than no preamp?

honashagen, The preferred hookup for the Pass Labs with it's super
symmetry circuitry is via XLR, since your amp doesn't accept balanced interconnects you won't be optimizing the capabilities of the Pass Labs pre. The McCormack is a fine amp and perhaps even the ideal match for your system, but it seems to be the bottleneck for what your trying to achieve. Perhaps the money spent on the Pass pre might have better been spent on an amp that plays nice with the Oppo?

Agree with fsonicsmith re jump factor. 

Answering the OP question, I'd say $5k to $20k. In other words, the price of a Shindo preamp. 

I would rather go straight from DAC to amp using jriver digital volume control than use a $2k preamp. The big jump, where the preamp adds musicality (and not just warmth) happens above $5k. 

Warmth is often just smearing and roll off. Musicality is extra tone, no details smeared away, and extra compelling transients. 

Buy the New Schiit preamp at $699 using 6sn7  tubes, 
justreplace them with the Golden Lions ,and you have the =
to preamps up to 3 timesthe cost, even a solid state option if you choose to  but a Pangea sig, MK-2 power cord. Fantastic for under$900 maxed out .also upgrade the fuse if you choose to.

$5500!

Happy Listening

Find an Ayre K3x with phono. Why waste more money for little gain. Because it is new and fancy, doesn't necessarily mean you will get that exponential gain in listening pleasure.Use your spare cash on something else worthwhile to you. How many enthusiasts have "aging" componentry that cannot be replaced because yesterday's stuff was quality, todays is science and mass market, or hi-brow if you can afford it.